[Roger Thornhill's matchbook carries the initials RoT.]
Eve Kendall: What does the O stand for?
Roger Thornhill: Oh, nothing.
In the shooting scene in the Mount Rushmore cafe, a boy in the background puts his fingers in his ears, because he knows the gun is about to be shot.
In the scene when Cary Grant is going up the steps to the UN, Alfred Hitchcock shot it from a rug truck across the street. He wasn't allowed to shoot the front of the UN. If you look closely, you can see a security guard in the left corner.